Q-switched all-fiber laser

ABSTRACT

A Q-switched all-fiber laser has a long period fiber grating (LPFG) switch with optical spectral characteristics that are controlled by application of stress. An actuator applies stress to selected sections of the LPFG in order to switch a fiber laser cavity at a specified wavelength. A controller controls the application of stress in the time domain, thereby switching the Q-factor of the fiber laser cavity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/022,255, entitled Q-SWITCHED ALL-FIBER LASER, filed Jan. 30,2008, which claims the benefit of priority to U.S. Provisional PatentApplication 60/901,255, filed Feb. 13, 2007, entitled Q SWITCHED FIBERLASER WITH ALL FIBER CONFIGURATIONS, both of which are incorporated byreference.

BACKGROUND

Characteristic features of fiber lasers include high output beamquality, compact size, ease-of-use, and low running cost. Fiber laserscan generate either continuous-wave (CW) radiation or pulse radiation.Pulsed operation can be achieved via Q-switching techniques. Q-switchedfiber lasers are preferred for applications such as micro-machining,marking, and scientific research due to their high peak power andexcellent beam quality. Q-switching is achieved by inserting a fastoptical switch in the laser resonance cavity to control optical loss inthe cavity. In particular, the optical switch must have fast speed, lowinsertion loss and high switch extinction ratio. Initially, the lasercavity is kept on a low Q factor state, laser oscillation cannot occurat this initial period, but energy from a pump source accumulates in thegain medium. Subsequently, the laser cavity is switched to a high Qfactor state, so that laser oscillation builds up quickly in the cavityand generates a high peak power laser pulse. When the laser cavity isswitched between low Q and high Q by the optical switch, sequenced laserpulses are produced.

Optical switching for Q-switching can be achieved by either active orpassive means. Examples of active Q-switching means includeacousto-optic modulators (AOMs) and electro-optic modulators (EOMs). TheAOM comprises optical crystals such as tellurium dioxide, crystallinequartz, and fused silica. The EOM comprises optical materials such aspotassium di-deuterium phosphate (KD*P), beta barium borate (BBO),lithium niobate (LiNbO₃), as well as NH₄H₂PO₄ (ADP), and othermaterials. One drawback of known AOM and EOM devices is that they arerelatively bulky. This is a drawback particularly for Q-switching infiber laser system, because the fibre core has a relatively smalldiameter, the difference of which relative to the size of the modulatorcomplicates light coupling between the device and an optical fiber.Further, AOM and EOM devices are relatively expensive.

A typical configuration of a Q-switched fiber laser is illustrated inFIG. 1. The laser cavity comprises a pair of fiber Bragg grating (FBG)reflectors (15, 35) having the same center wavelength, a gain fiber (18)which provides optical gain, and an optical switch (90) coupled to anoptical fiber pigtail (20) for coupling a light signal between the fiberand the switch. The optical switch may be either an AOM or EOM type. Apump source (1) provides pump light (5) which is coupled to the fiberlaser cavity to excite the gain fiber (18). The FBG reflectors provideoptical feedback for laser oscillation. The optical switch (90) isemployed as a switch to control optical loss within the laser cavity,and thereby provide Q-switching. Initially, the cavity loss is kept on ahigh level with the switch “off” (low Q factor state of the lasercavity), at which time no light signal passes through the switch (90).As discussed above, laser oscillation does not occur at this time, butenergy from pump light source (5) accumulates in the gain fiber (18).Subsequently, the cavity loss is reduced over a relatively short time by“switching on” the optical switch to a low loss level (high Q factorstate of the laser cavity), at which time the light signal passesthrough optical switch (90). Consequently, laser oscillation builds upquickly in the cavity and generates a high peak power laser pulse. TheFBG pair (15, 35) have the same center wavelength and function as narrowband reflective mirrors which provide optical feedback to the lasercavity and confine the laser oscillation wavelength to the FBGwavelength. Since the FBG has a relatively narrow reflective bandwidth,the laser oscillates only at this wavelength and the output has a narrowwavelength spectrum. When the laser cavity is switched sequentiallybetween the low Q factor state and the high Q factor state by means ofthe optical switch (90), sequenced laser pulses are produced. Switchcontrol is achieved by means of a signal (95) from an externalcontroller (96). One device of the FBG pair (15, 35) is partiallytransparent and has relatively lower reflectivity, resulting in apercentage of the generated laser light being permitted to leave lasercavity and deliver the laser output (38 or 42).

Referring to FIG. 2 a, the FBG is formed by introducing a periodicchanges of refractive index in the fiber core. The modified area (151)within the fiber core has a smaller refractive index difference ofperiod Λ_(B) relative to the adjacent unmodified area (152). Severaltechniques are known for changing the refractive index of discreet areasof the fibre core. One technique is to expose the area to a UV laserbeam, e.g., area (151) is altered by exposure to UV light, but area(152) is neither exposed nor altered.

The principle characteristic parameters of a FBG are center wavelengthλ_(B), bandwidth Δλ_(B), and reflectivity. The condition for highreflection, known as the Bragg condition, relates the reflectedwavelength, or Bragg wavelength, λ_(B) to the grating period Λ_(B) andthe effective refractive index of the fiber core n via:λ_(B)=2nΛ _(B).FIGS. 2 b, 2 c, and 2 d illustrate the spectral characteristics of aFBG. When broad band light (110, FIG. 2 a) having spectrum (120, FIG. 2b) is input into the FBG as shown, the reflected light (112, FIG. 2 a)has a corresponding spectrum (122, FIG. 2 c), and the transmitted light(111, FIG. 2 a) has a corresponding spectrum (121, FIG. 2 d).

Somewhat similar to the FBG in terms of physical configuration, a LongPeriod Fiber Grating (LPFG) has a grating period Λ_(L) which isconsiderably longer than the period Λ_(B) of the FBG, i.e., typicallyΛ_(L) is 200˜2000 times longer than Λ_(B). The LPFG couples thefundamental mode in the fiber core with the cladding modes of the fiberand propagates them in the same direction. The excited cladding modesare attenuated, resulting in the appearance of resonance loss in thetransmission spectrum. However, in contrast with the FBG, the LPFG doesnot produce reflected light. FIGS. 3 a, 3 b and 3 c illustrate thephysical configuration and the spectral transmission characteristics ofa LPFG. The periodic grating structure (22, FIG. 3 a) can be made byusing a UV laser beam to “burn” discreet, periodically spaced areas inthe fiber core in a manner which is similar to that described above withreference to the FBG, where the modified area (251) exhibits arefractive index change in comparison with unmodified area (252). Recentresearch suggests that the modified areas can be also formed by using ahigh voltage electric arc discharge or CO₂ laser to “burn” the fiber,i.e., introducing structural changes and slight geometrical deformationin the irradiated area of the fibre. Mechanical stress can be used,e.g., by applying static stress to the areas of the fibre to be modifiedthrough a corrugated plate. The refractive index at the areas subjectedto stress is changed in accordance with the photo-elastic effect, butthe adjacent areas which are not subjected to stress are unmodified.

When a broad band light (210, FIG. 3 a) having spectrum (220, FIG. 3 b)is input into the LPFG, the transmitted light (211, FIG. 3 a) has acorresponding spectral characteristic (221, FIG. 3 c), several resonanceloss peaks (222, 223), including the fundamental mode coupling withdifferent cladding modes of the fiber. However, there is no lightreflection. Considering resonance loss peak (222, FIG. 3 c), having acenter wavelength XL, and bandwidth Δλ_(L), the resonance loss of theLPFG is due to the coupling of the fundamental mode in the fiber corewith the cladding modes of the fiber. The phase matching between thefundamental mode and cladding modes at wavelength λ_(mL) can beexpressed as:λ_(mL)=(n _(core) −n _(cl) ^(m))Λ_(L),where n_(core) is the effective refractive index of the fundamentalmode, n_(cl) ^(m) is the effective refractive index of the m^(th)cladding mode, and Λ_(L) is the period of the LPFG. Since severalcladding modes can satisfy this condition, each one is at a differentcenter wavelength λ_(mL), and thus the transmission spectrum of the LPFGexhibits a series of transmission loss notch peaks (222, 223, FIG. 3C).

FIGS. 4 a-4 c illustrate the physical configuration and the spectraltransmission characteristics of a phase shifted LPFG. In the phaseshifted LPFG, a part of the grating period is shifted at the gratingcenter by Λp. As a result, a phase shift is introduced into the LPFG.For example, by introducing a π-phase shift at the center of the LPFG,the notch peak (See FIG. 3 c) is changed to a reverse peak (232, FIG. 4c). For a broad band input (220, FIG. 4 b), a corresponding transmissionspectrum (231, FIG. 4 c) of the phase shifted LPFG is produced, enablingtransmission at wavelength XL.

FIGS. 5 a-5 c illustrate the physical configuration and the spectraltransmission characteristics of cascaded LPFGs. Cascaded LPFGs areformed by connecting a pair of LPFGs (25, 26) in series. Each of theLPFGs has a grating length d₁ and d₂, and together define a separationdistance of L. When broad band light (210) having spectrum (220, FIG. 5b) is input into the cascaded LPFGs, the corresponding transmitted light(211) has a corresponding spectral transmission response (241, FIG. 5c). It can be seen from FIGS. 5 b and 5 c that the spectrum of thetransmitted light has several spectral transparent peaks (242, 244 and246) and several spectral loss peaks (245, 243). This is due tointerference between the fundamental mode and cladding modes. The firstLPFG couples part of the fundamental mode to the cladding modes, andthen the coupled cladding modes and fundamental mode travel along thefiber simultaneously to the second LPFG. At the second LPFG, the twomodes interact with each other and generate spectral interference fringepatterns. The fringe spacing Δλ_(PL) is related to the grating lengthd₁, d₂, d and the separation distance L between the two LPFGs. Anincrease in L corresponds with a decrease in the fringe spacing Δλ_(PL).For multi-channel filter applications the distance L is typically lessthan 600 mm.

SUMMARY

All examples and features mentioned below can be combined in anytechnically possible way.

In accordance with one aspect a fiber laser apparatus configured togenerate Q switched optical pulses comprises: a first fiber Bragggrating having a first reflective center wavelength; a second fiberBragg grating having a second reflective center wavelength, the secondreflective center wavelength being in a reflective wavelength bandwidthof the first fiber Bragg grating, the first and second fiber Bragggratings configured to provide optical feedback for laser oscillationand to confine laser oscillation wavelength; at least one long periodfiber grating disposed between the first fiber Bragg grating and thesecond fiber Bragg grating, the long period fiber grating having aswitching wavelength in which switching wavelength bandwidth fullycovers the wavelength bandwidth of the first fiber Bragg grating and thesecond fiber Bragg grating; a gain fiber disposed between the firstfiber Bragg grating and the second fiber Bragg grating, the gain fiberconfigured to provide optical gain; and at least one actuator configuredto apply stress to the long period fiber grating to cause switching of Qfactor of the laser cavity and generation of Q switched optical pulses.In some implementations the actuator causes temporary deformation of thelong period fiber grating, resulting in a change of effective refractiveindex. In some implementations the apparatus further comprises acontroller which controls the actuator such that the stress is appliedin a periodic time varying manner. In some implementations the actuatorapplies the stress acoustically. In some implementations the apparatusfurther comprises an optical fiber selected from a group including:normal fiber, double clad fiber, photonic crystal fiber, polarizationmaintaining fiber and rare earth doped fiber. In some implementationsthe at least one long period fiber grating is selected from a groupincluding: a normal long period fiber grating, a phase shifted longperiod fiber grating and a cascaded long period fiber grating.

In accordance with another aspect a fiber laser apparatus configured togenerate Q switched optical pulses comprises: a fiber Bragg gratingreflector configured to provide optical feedback for laser oscillationand to confine laser oscillation wavelength; a fiber loop mirror formedby splicing first and second distal ends of a fiber coupler; at leastone long period fiber grating disposed between the fiber Bragg gratingreflector and the fiber loop mirror, the long period fiber gratinghaving a switching wavelength in which a switching wavelength bandwidthfully covers a wavelength bandwidth of the fiber Bragg grating; a gainfiber configured to provide optical gain; and at least one actuatorconfigured to apply stress to the long period fiber grating to causeswitching of Q factor of the laser cavity and generation of Q switchedoptical pulses. In some implementations the actuator causes temporarydeformation of the long period fiber grating, resulting in a change ofeffective refractive index. In some implementations the apparatusfurther comprises a controller which controls the actuator such that thestress is applied in a periodic time varying manner. In someimplementations the actuator applies the stress acoustically. In someimplementations the apparatus further comprises an optical fiberselected from a group including: normal fiber, double clad fiber,photonic crystal fiber, polarization maintaining fiber and rare earthdoped fiber. In some implementations the at least one long period fibergrating is selected from a group including: a normal long period fibergrating, a phase shifted long period fiber grating and a cascaded longperiod fiber grating.

In accordance with another aspect a fiber laser apparatus configured togenerate Q switched optical pulses comprises: a fiber Bragg gratingreflector configured to provide optical feedback for laser oscillationand to confine laser oscillation wavelength; a fiber loop mirror formedby splicing first and second distal ends of a fiber coupler; at leastone long period fiber grating disposed in the fiber loop mirror, thelong period fiber grating having a switching wavelength in which aswitching wavelength bandwidth fully covers a wavelength bandwidth ofthe fiber Bragg grating reflector; a gain fiber configured to provideoptical gain; and at least one actuator configured to apply stress tothe long period fiber grating to cause switching of Q factor of thelaser cavity and generation of Q switched optical pulses. In someimplementations the actuator causes temporary deformation of the longperiod fiber grating, resulting in a change of effective refractiveindex. In some implementations the apparatus further comprises acontroller which controls the actuator such that the stress is appliedin a periodic time varying manner. In some implementations the actuatorapplies the stress acoustically. In some implementations the apparatusfurther comprises an optical fiber selected from a group including:normal fiber, double clad fiber, photonic crystal fiber, polarizationmaintaining fiber and rare earth doped fiber. In some implementationsthe at least one long period fiber grating is selected from a groupincluding: a normal long period fiber grating, a phase shifted longperiod fiber grating and a cascaded long period fiber grating.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates the physical configuration of a Q-switched fiberlaser.

FIGS. 2 a, 2 b, 2 c and 2 d illustrate the physical configuration andspectral transmission characteristics of a FBG.

FIGS. 3 a, 3 b, and 3 c illustrate the physical configuration andspectral transmission characteristics of a LPFG.

FIGS. 4 a, 4 b, and 4 c illustrate the physical configuration andspectral transmission characteristics of a phase shifted LPFG.

FIGS. 5 a, 5 b, and 5 c illustrate the physical configuration andspectral transmission characteristics of a cascaded LPFG pair.

FIGS. 6 a, 6 b, and 6 c illustrate a LPFG optical switch.

FIG. 6 d and FIG. 6 e illustrate the spectral transmission behavior ofthe innovative LPFG switch.

FIG. 7 illustrates use of the LPFG switch as a component of an all-fiberin-line device such as an all-fiber Q-switched laser.

FIGS. 8 a, 8 b, and 8 c illustrate an LPFG switch formed by employing aphase shifted LPFG.

FIG. 9 a illustrates an implementation of the switch based on cascadedLPFGs.

FIGS. 9 b, 9 c, 9 d and 9 e illustrate the spectral transmissioncharacteristics of the implementation of FIG. 9 a.

FIG. 10 illustrates a Q-switched fiber laser employing two LPFG switchin the fiber laser cavity.

FIGS. 11 a, 11 b, and 11 c illustrate an LPFG switch assembly using twoLPFGs.

FIG. 12 illustrates a Q-switched fiber laser employing a LPFG switch inwhich the pump light is coupled into the laser cavity from the middle ofthe laser cavity.

FIG. 13 illustrates a Q-switched fiber laser employing a LPFG switch inwhich a ring laser cavity is used and the LPFG switch is placed outsideof the fiber loop.

FIG. 14 illustrates a Q-switched fiber laser employing a LPFG switch inwhich a ring laser cavity is used and the LPFG switch is placed insideof the fiber loop.

FIG. 15 illustrates a Q-switched fiber laser in which a LPFG switch isemployed in a ring laser cavity and an optical isolator is employed toachieve unidirectional laser oscillation.

FIG. 16 illustrates a Q-switched fiber laser having a ring laser cavityin which the pump light is coupled into the laser cavity from the middleof the cavity.

DETAILED DESCRIPTION

Referring to FIGS. 6 a-6 c, a LPFG optical switch is provided via thecontrolled (time, area and force) application of stress to an opticalmaterial to introduce refractive index changes in the material inaccordance with the photo-elastic effect. As illustrated, a smallsection (253) of the LPFG (22) is subjected to stress (203) throughforce applied by an actuator (202). The stress may be applied bymechanical, acoustic or other means. The actuator (202) may include apiezo actuator that operates in response to a switching voltage (205)from a controller (206). The applied stress (203) causes a temporarydeformation of the material at section (253) and a correspondingrefractive index change at section (253). The periodic structure andspectral transmission behavior of the LPFG are changed in acorresponding manner. In particular, the magnitude of the refractiveindex change is related to the magnitude of applied force, the periodicstructure and spectral transmission behavior is related to (a) whichareas are subjected to stress and (b) the period and frequency at whichstress is applied.

FIGS. 6 b and 6 c are cross-sectional views of the LPFG of FIG. 6 a thatillustrate different configurations for applying stress to the fibre(20). In FIG. 6 b the LPFG fiber (20) is disposed between actuator (202)and a plate (215). The fiber can be fixed in place with glue (207). FIG.6 c shows an implementation in which a V-groove plate (216) is employedin lieu of the flat plate (215, FIG. 6 b) for enhanced fiber fixing andenhanced stress distribution.

FIG. 6 d and FIG. 6 e illustrate the spectral transmission behavior ofthe innovative LPFG switch. The transmission spectrum of the LPFG whenno stress is applied is shown by a first section (221, FIG. 6 d), i.e.,a narrow band input light (122) with center wavelength λ_(L) is blockedsince the resonance loss peak (222) of the LPFG is just at thiswavelength. This corresponds to the “switch off” state of the LPFGswitch. The bandwidth of the signal light is narrower than the bandwidthΔλ_(L) of the LPFG. When stress is applied to section (253, FIG. 6 a),the transmission spectrum is changed as shown in FIG. 6 e, with theresonance loss peak (222, FIG. 6 d) becoming peak (222 a, FIG. 6 e). Thenarrow band input light (122) can now pass through the LPFG. Thiscorresponds to the “switch on” state of the LPFG switch. Thus, the inputlight (122) with center wavelength λ_(L) can be switched in response tothe control signal applied to the actuator.

FIG. 7 illustrates use of the LPFG switch as a component of an all-fiberin-line device such as an all-fiber Q-switched laser. The illustratedlaser cavity has a Fabry-Perot configuration and includes a pair of FBGreflectors (15, 35) having the same center wavelength λ_(B), a gainfiber (18), and an LPFG switch (201). The resonance loss peak λ_(L) ofthe LPFG is matched with center wavelength λ_(B) of the FBGs. Thebandwidth Δλ_(B) of the FBGs is narrower than bandwidth Δλ_(L) of theLPFG, i.e., Δλ_(B)<<Δλ_(L). The laser oscillation wavelength is confinedby the FBGs at wavelength λ_(B). The LPFG switch is employed to switchthe Q factor of the laser cavity, i.e., control optical loss in the timedomain. Switching is provided in response to a switching voltage (205)applied to the actuator by a controller (206). Pump source (1) couplespump light (5) into the laser cavity to pump gain fiber (18). One orboth of the FBG reflectors (15, 35) are partially transparent at itswavelength. Consequently, the laser output (38 or 42) can be providedfrom either fiber end (37) or fibre end (9), or both fiber ends.

An LPFG switch can be formed by employing a phase shifted LPFG as shownin FIG. 8 a. In this implementation the stress (203) is applied to thephase shift section on the LPFG through actuator (202). The transmissionspectrum of the phase shifted LPFG with and without applied stress isshown in FIG. 8 b and FIG. 8 c. When no stress is applied to the LPFG, anarrow band signal light (122) can pass through the area (232) of thephase shifted LPFG, i.e., in the “switch on” state. When stress isapplied to area (253) the LPFG has resonance loss (232 a) at wavelengthλ_(L), i.e., in the “switch off” state. As with the previousimplementation, the bandwidth of the signal light is narrower thanbandwidth Δλ_(L) of the LPFG.

FIG. 9 a illustrates an implementation of the switch based on cascadedLPFGs. A pair of LPFGs (25, 26) are disposed in series. Actuator (202 or202 b or 202 a) applies stress to the section of LPFG (25) or LPFG (26)or on the fiber section (227) between LPFG (25) and LPFG (26).Initially, when no stress is applied, the transmission spectrum is asshown at section (241) in FIG. 9 b. The wavelength of the signal light(122) is matched at the wavelength Δλ_(L1), which is at loss peak (243)on the spectrum of the cascaded LPFGs. Consequently, the signal light(122) cannot pass through and the switch is in the “switch off” state.When the stress is applied at any of points (202, 202 b or 202 a), thetransmission spectrum is changed as shown in FIG. 9 c, where the signallight (122) can pass through since Δλ_(L1) at peak (243 a) istransparent.

FIGS. 9 d and 9 e illustrate an implementation in which, when no stressis applied, the signal light (122) can pass through the cascaded LPFGssince the wavelength of the signal light is set to match λ_(L2) at (244,FIG. 9 d). When stress is applied, the signal light (122) is blockedsince the spectrum of the cascaded LPFGs is changed as shown in FIG. 9 ewhere the signal light (122) is at the loss peak (244 a) in the spectrumof the cascaded LPFGs.

Generally, any of the LPFG switches described above can be utilized toprovide an all-fibre Q-switched laser. FIG. 10, for example, illustratesan implementation of the Q-switched laser in which two LPFG switches(201 a, 201 b) are employed in the fiber laser cavity to enhance switchextinction. Two or more LPFGs can also be packaged together as shown inFIGS. 11 a, 11 b and 11 c. The fibers (262, 271) with LPFGs (265, 275)are sandwiched between actuator (202) and plate (215) or V-groove (216).Again, glue (207) may be used to protect and fix the fiber. Switchingvoltage (209) is applied to actuator (202). FIG. 12 illustrates animplementation of the Q-switched fiber laser system in which the pumplight is coupled into the fiber laser cavity from the middle of thelaser cavity. In particular, the pump light (5) is coupled into lasercavity through pump coupler (4). The Q-switched fiber laser can also beimplemented with ring laser cavity configurations as shown in FIG. 13.The laser cavity comprises FBG reflector (15), LPFG switch (201), fibercoupler (60) and gain fiber (18). Two arms (62, 64) of the fiber coupler(60) are spliced with gain fiber (18) to form a fiber loop. The LPFGswitch (201) is placed outside of the fiber loop between the FBG (15)and the fiber coupler (60). The LPFG switch is transparent at the pumpwavelength, and the resulting laser output comes from the arm (63) ofthe fiber coupler. FIG. 14 illustrates another possible implementationof the ring fiber laser cavity where the LPFG switch (201) is placedinside the fiber loop. Furthermore, the gain fiber can be placed outsideof the fiber loop. In this case the fiber loop forms a fiber loopmirror. FIG. 15 illustrates another implementation of the ring fiberlaser cavity in which an isolator (70) is placed in the fiber loop inorder to achieve unidirectional laser oscillation in the laser cavity.The LPFG switch (201) can be placed either in the fiber loop or outsideof the fiber loop between the FBG (15) and the fiber coupler (60). FIG.16 illustrates an implementation of the LPFG switch based Q-switchedfiber laser having ring laser cavity in which the pump light is coupledinto the laser cavity from the middle of the cavity through the pumpcoupler (4). In any of the implementations of the Q-switched fiber laseremploying an LPFG switch, the LPFG switch may be a simple LPFG basedswitch, a phase shifted LPFG based switch or a cascaded LPFGs basedswitch. Further, one or more LPFG switches may be used in a fiber lasercavity in order to improve switch extinction.

A number of implementations have been described. Nevertheless, it willbe understood that additional modifications may be made withoutdeparting from the scope of the inventive concepts described herein,and, accordingly, other features, aspects and implementations are withinthe scope of the following claims.

What is claimed is:
 1. A fiber laser apparatus configured to generate Qswitched optical pulses, comprising: a first fiber Bragg grating havinga first reflective center wavelength; a second fiber Bragg gratinghaving a second reflective center wavelength, the second reflectivecenter wavelength being in a reflective wavelength bandwidth of thefirst fiber Bragg grating, the first and second fiber Bragg gratingsconfigured to provide optical feedback for laser oscillation and toconfine laser oscillation wavelength; at least one long period fibergrating disposed between the first fiber Bragg grating and the secondfiber Bragg grating, the long period fiber grating having a switchingwavelength in which a switching wavelength bandwidth fully covers thewavelength bandwidth of the first fiber Bragg grating and the secondfiber Bragg grating; a gain fiber disposed between the first fiber Bragggrating and the second fiber Bragg grating, the gain fiber configured toprovide optical gain; and at least one actuator configured to applystress to the long period fiber grating to cause switching of Q factorof the laser cavity and generation of Q switched optical pulses.
 2. Thefiber laser of claim 1 wherein the actuator causes temporary deformationof the long period fiber grating, resulting in a change of effectiverefractive index.
 3. The fiber laser of claim 2 further comprising acontroller which controls the actuator such that the stress is appliedin a periodic time varying manner.
 4. The fiber laser of claim 3 whereinthe actuator applies the stress acoustically.
 5. The fiber laserapparatus of claim 1 further comprising an optical fiber selected from agroup including: normal fiber, double clad fiber, photonic crystalfiber, polarization maintaining fiber and rare earth doped fiber.
 6. Thefiber laser apparatus of claim 1 wherein the at least one long periodfiber grating is selected from a group including: a normal long periodfiber grating, a phase shifted long period fiber grating and a cascadedlong period fiber grating.
 7. A fiber laser apparatus configured togenerate Q switched optical pulses, comprising: a fiber Bragg gratingreflector configured to provide optical feedback for laser oscillationand to confine laser oscillation wavelength; a fiber loop mirror formedby splicing first and second distal ends of a fiber coupler; at leastone long period fiber grating disposed between the fiber Bragg gratingreflector and the fiber loop mirror, the long period fiber gratinghaving a switching wavelength in which a switching wavelength bandwidthfully covers a wavelength bandwidth of the fiber Bragg grating; a gainfiber configured to provide optical gain; and at least one actuatorconfigured to apply stress to the long period fiber grating to causeswitching of Q factor of the laser cavity and generation of Q switchedoptical pulses.
 8. The fiber laser of claim 7 wherein the actuatorcauses temporary deformation of the long period fiber grating, resultingin a change of effective refractive index.
 9. The fiber laser of claim 8further comprising a controller which controls the actuator such thatthe stress is applied in a periodic time varying manner.
 10. The fiberlaser of claim 9 wherein the actuator applies the stress acoustically.11. The fiber laser apparatus of claim 7 further comprising an opticalfiber selected from a group including: normal fiber, double clad fiber,photonic crystal fiber, polarization maintaining fiber and rare earthdoped fiber.
 12. The fiber laser apparatus of claim 7 wherein the atleast one long period fiber grating is selected from a group including:a normal long period fiber grating, a phase shifted long period fibergrating and a cascaded long period fiber grating.
 13. A fiber laserapparatus configured to generate Q switched optical pulses, comprising:a fiber Bragg grating reflector configured to provide optical feedbackfor laser oscillation and to confine laser oscillation wavelength; afiber loop mirror formed by splicing first and second distal ends of afiber coupler; at least one long period fiber grating disposed in thefiber loop mirror, the long period fiber grating having a switchingwavelength in which a switching wavelength bandwidth fully covers awavelength bandwidth of the fiber Bragg grating reflector; a gain fiberconfigured to provide optical gain; and at least one actuator configuredto apply stress to the long period fiber grating to cause switching of Qfactor of the laser cavity and generation of Q switched optical pulses.14. The fiber laser of claim 13 wherein the actuator causes temporarydeformation of the long period fiber grating, resulting in a change ofeffective refractive index.
 15. The fiber laser of claim 13 furthercomprising a controller which controls the actuator such that the stressis applied in a periodic time varying manner.
 16. The fiber laser ofclaim 14 wherein the actuator applies the stress acoustically.
 17. Thefiber laser apparatus of claim 13 further comprising an optical fiberselected from a group including: normal fiber, double clad fiber,photonic crystal fiber, polarization maintaining fiber and rare earthdoped fiber.
 18. The fiber laser apparatus of claim 13 wherein the atleast one long period fiber grating is selected from a group including:a normal long period fiber grating, a phase shifted long period fibergrating and a cascaded long period fiber grating.